The present invention relates to semiconductor fabrication. In particular, it addresses certain problems in making polysilicon resistors in a polycide line. "Polycide" is the term of art given to a multi-layered structure having a silicide, such as titanium silicide or tungsten silicide, on top of polysilicon.
Polysilicon can have a low resistance or a high resistance or resistivity, depending on how it is doped, and in current semiconductor technology, resistors often are fabricated in a polysilicon layer. The resistor can be in a word line or other line which has other parts which should be highly conductive. Thus, polycide, which can be very conductive, is used for lines in such semiconductor devices as memories. The resistor can be formed in the polysilicon part of the polycide.
A problem develops in that if one simply forms a polycide layer consisting of polysilicon covered with a silicide, and then etches off the silicide in a selective fashion to expose areas which are to be resistors, then the resistors do not, it has been observed, come out uniformly. Indeed, there are some resulting resistors which can fairly be called freaks. Evidently, there is some interaction between the silicide and the polysilicon which leads to an adverse effect.
Thus, it is the primary object of the present invention to develop a process for fabricating a reliable polysilicon resistor in a polycide environment.
Another object of the present invention is to allow the use of low resistivity silicide and high resistivity polysilicon in the same layer.